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Undercut Footings


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First, underminement isn't a word, or at least it isn't in my dictionary.

Digging out under a footing is bad. Shouldn't do it.

Honest, though, it's a real common practice around here when digging out bsmt's. for added square footage. Heck, I've dug under the footing of my solid load bearing masonry 1 1/2 story bungalow a width of 3 feet to run a sewer line; it works fine. You won't catch me recommending it to customer's, but it happens all the time.

I see lots of jobs where the damn footing is completely removed, and the house hangs there by skyhook, while the contractor excavates, places a new footing, then forms up and places a new concrete foundation under the old one to bear on the new footing. Sounds scary, doesn't it? It's not, but it ain't for the faint hearted.

They're just single family houses. We used to build them up, tear them down, move walls around, rebuild foundations, and do all sorts of stuff that might make the inexperienced freak out and call for review by a structural engineer, but years ago, everyone understood how houses work, and no one ever called an engineer for anything, and nothing unexpected or bad ever happened.

I wish I had an inspection school. I'd have an old house, and we'd take a week taking out load bearing walls, cutting holes in the foundation, or otherwise doing all sorts of evil stuff to see what actually happens, and to experience viscerally what forces are at work. I think most folks would be surprised how simple it really is. Nothing falls down quickly; stuff falls down very, very slowly. A little alteration here or there isn't going to mean a hill of beans in most cases.

That said, if I saw your picture condition on an inspection, I'd certainly write it up as needing "repair", mostly because there's always some contractor to come in and tell folks there house is going to fall down if they don't fix the footing. (didn't your inspector tell you about this!!??!)

I'd do it to cover my butt. On a practical level, though, I don't think that little bit of missing footing is going to do anything other than just sit there like it is now.

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I don't make an issue of a single point excavation like that.

I'm not an engineer but common sense tells me that the concrete stem wall is, in essence, spanning or bridging the gap--it's a mini bridge. Concrete's very strong in compression like that; even more so if there's re-steel inside.

Lots of sump pump and gravel-filled trenches in these parts. If I see one where they've dug too close to the "cone of compression" (I've never used that term before, but its cool) I'll write it and recommend they have it fixed. That type of work also implies the people that did the gravel drain system probably don't have much experience either.

How do you fix a continuously undermined section of footing? Not sure. Its not like you can just dump some soil back in and get the necessary compaction with a machine in a tight space like that. Anyone else?

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Very sage, Kurt. Very sage.

One of the beams in my eighty-five year old bungalow had been completely decimated by termites, but end result? The floors were a little out of kilter and a couple of the door jambs were an inch or so out of square. I've since reinforced everything to make certain stasis is attained, but the termites took their best shot and the old girl has held up pretty well.

Chris, I don't think there really is a feasible repair for the footer in your photo, at least not one that would actually provide any support. Unless, of course, a structural repair company injected concrete to serve as a pier beneath the footer. But that would be totally unnecessary.

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Originally posted by Chris Bernhardt

Thanks Kurt and Randy.

But does anyone know the proper way to correct it?

Can you just rebuild it back up compacting it by hand or do you dig it out and pour more concrete?

Chris, Oregon

You can pump concrete in there, or even slurry.

For a small section, you can dry-pack it with grout. Works great.

Alternatively, you can fill the hole with clean sharp rock or river rock. It'll fill the void, it won't settle and it'll prevent the sides of the opening from eroding.

But if it's just a small section, 2-3 feet say, I wouldn't bother fixing it. It just isn't a real problem.

Now if someone's excavated beside and below the footing for a significant distance, that's a problem. Particulary if they've done it to accommodate a drain tile. The soil under the footing will slowly erode away. (Are you familiar with the term, "mass wasting?")

-Jim Katen, Oregon

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Most old-house basements here were originally 5'-6'. Many are later excavated, well below the original wall (no footings here). There is a satisfactory method of retaining the soil below the foundation. Google-up "foundation benching".

Or you can hire a waterproofer to make sure that every last bit of earth gets pulled out from under the foundation.

20071025161018_underminement.jpg

Chris, thanks for the new vocab - I'm going to use it in my new boilerplate.

"Underminement of the footing can effect the stabilizidity of the foundation, resulting in settlementatude".

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I wouldn't call for an engineer for a problem like Chris posted in the top photo. I'd call a foundation contractor to fix it. Jim covered the repair methods pretty well.

While a small section of the foundation that bears an evenly distributed load might not be a problem, if the void was below a load path that was applying a concentrated point load on the foundation from above, it could be trouble.

Excavating in the cone of compression along a footing is dangerous stuff. Depending upon the soil type and moisture content, it could give away suddenly causing a collapse. It'll look fine right up until the time that it isn't.

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Originally posted by inspecthistoric

Or you can hire a waterproofer to make sure that every last bit of earth gets pulled out from under the foundation.

That looks like what I was talking about; it looks like all the soil is removed from under the foundation. Is that what the pic shows?

Why doesn't it collapse?

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Originally posted by Al Austin

An engineer should be consulted at any time a serious foundation problem is visually observed

Visually observed? Is there another way you would observe it? Passive voice twice in one sentence.

An engineer is going to charge a few hundred bucks and tell you to call a foundation repair company.

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Using my little engineering brain I agree with Jim K. I can't see why, assuming that the errosion is simple, one couldn't just recompact the ground underneath and adjacent to the footer with dry concrete and gravel and even earth?

Now if there were clues that the ground was settling under the footer that would be a different story.

Has anyone here ever been told that recompacting this way is a bad Idea?

Chris, Oregon

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A couple of years ago I had to underpin a section of foundation about 40' long. We were dropping a basement floor about 2 feet.

I dug out an approximatly 2 foot sections spaced approximatly 6 feet apart and well past the exterior of the foudation. The hole extended about 3 feet lower than the new floor level and was "belled" on the bottom.

I then formed in the void and poured concrete high enough to encapsulate the old foundation by about 12 inches.

One section of the old foundation had collapsed (in prior years), I jacked up that section prior to encasing it in the form.

A few weeks later after stripping the forms I dug out in between my retrofitted footings. Although at these locations I did not excavate quite as much. I then built forms and underpinned the remaining areas.

All held very well.

Chad, don't you mean settlementatudation?

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Here is a blanket answer to many foundation failures.

Pages 451 thru 462 (and in some instances extending to 484) of an appropriate industrial text deals very clearly with the subject matter. An engineer or technician familiar with the topic of footing and soil mechanics would easily know which text is referenced.

A collection of the empirical data is necessary to make a 3D detail load and bearing diagram. The variables need to be collected to fit them into the mathematical model. That takes an orderly approach. Accurate modeling enhances predictability. Time line reconstruction can also map out a history.

A drainage / soil type, density and moisture survey could also be done. Some engineers will give an honest estimate of the repairs based on a variety of factors. That estimate and analysis should be untainted by the usual bias from a foundation contractor or other third party.

Companies that do wide-ranging analysis commercial property work usually have an engineer on staff or they outsource the work to an appropriate engineer with regional experience. Less viable residential properties and trades people just fall through the cracks. Pun intended.

In the instance of the property with the photos, it is obvious that repair work needs to be done. The manner and means in which it should be done is what is in question. The determination of who should perform any repairs and give opinions on the subject is also in question.

The specifics within the engineer’s report would help the potential buyer and contractors determine the best path of action. Anything less is unnecessarily risky and imprudent.

I would be interested in reading the engineer’s report. Without the collection of data and an analytical due diligence summation, the conclusions are perhaps only hopeful well-intentioned wags or swags.

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Originally posted by Al Austin

A collection of the empirical data is necessary to make a 3D detail load and bearing diagram. The variables need to be collected to fit them into the mathematical model. That takes an orderly approach. Accurate modeling enhances predictability. Time line reconstruction can also map out a history.

Not an attack, Al, an honest inquiry: I don't understand how that's possible on a foundation that's already in place, with a buliding on top of it. Aren't there too many variables? Would you please post your reference so I can read about it?

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  • 3 years later...
  • 3 weeks later...

As to undermining shown in photo.......which is relatively minor......simply fill in void with concrete.

Any such condition should always be noted in an inspection report.

To lower basement floor.....basic approaches are; (1) Bench wall, if loss of space for new (lower) floor is workable, or (2) Underpinning existing wall / footing with new concrete.

I have been called out on several foundation collapses caused by contractors exposing too much soil along foundation walls........trying to underpin a wall and installing a drain line. In one case, the contractor explained how, suddenly, a very large segment of thick stone wall "exploded" into the basement, nearly crushing himself and coworker.........he was obviously shaken and would not go back into the basement to install heavy timber shoring that was eventually installed (by others) to allow for major repair work.

In another case, the base of a long concrete foundation wall simply slipped inward and downward into the basement, breaking away near each end. Amazingly (though this happens), the wood-framed house wall remained intact, although it did sag about 2 inches. The wall essentially acts as a "deep beam" (at least for a while!)

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In my 34 years experience in structural engineering......including the last 18 as a consulting engineer.......I have found that (other than when a collapse occurs) it is quite difficult for the vast majority of those involved with buildings to grasp the reasons we "overdesign".

Also.........this "overdesign" tends to encourage practices (by contractors, owners and even architects and engineers) that counteract or "use up" the margin of safety, before it is needed.

Many contractors (and owners) take the approach that, as long as the building or structural element (component) is standing now....supporting only the weight of the building.........it must be fine. However that is the wrong standard for judging structural capacity.

In general, we design and construct buildings to last, literally, a lifetime (or more). We expect the building to support and resist not just "normal" everyday loads........but also extra-ordinary loads, such as, for example, 2 feet of snow on the roof.......a hurricane........crowds of party-goers packed tight on a deck.......or on a walkway in a Kansas City hotel (1981).......and even (at least in the US) earthquakes.

In the event of hurricanes and earthquakes......thousands of buildings could be destroyed at the same time.......with severe injury and death (see; Haiti earthquake).

On a more down-to-earth level for a particular house......many conditions that we assign to the "defect" category can be lived with.......sometimes for many years........because that defective element never has to be tested by conditions that we design for, based on long experience (over many years that can exceed the life experience of any single person) that shows ......one day........an extra-ordinary loading is bound to eventually occur.

The problem is that some of these defects will eventually have to face the test.........yet we can not predict where and when.

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